1. Field of the Invention
The present invention relates to a method of fabricating an internal composite layer composed of an electrically insulating substrate with a copper layer formed thereon and a film of a coupling agent covering the copper layer for a multilayer circuit board, which is capable of providing an increased adhesion between the coupling agent and the copper layer and excellent acid resistance of the internal composite layer.
2. Description of the Prior Art
In recent years, the demand of a composite consisting of an organic material and a metal is increasing in the field of an electric material or a structural material. However, since a thermal expansion coefficient of the organic material is much different from that of the metal, there is a possibility of causing a delamination between the organic material and the metal when the composite is used under the condition that it receives a heat cycle or a thermal shock. Therefore, it is desired to improve an adhesion strength between the organic material and the metal.
For example, a multilayer circuit board used for an electronic application consists of a number of internal circuit layers and resin impregnated sheets (prepregs) alternately laminated between external circuit layers or copper foils. Copper conductors are formed on one or both sides of the internal circuit layer by etching. Therefore, for providing an excellent multilayer circuit board, it is necessary for improving an adhesion strength between the copper conductors and the prepreg, or between the copper foil and the prepreg. It is known that the adhesion strength is improved in such a manner that a copper oxide (CuO) layer is formed on the copper conductors or the copper foil by a chemical oxidation method as described in U.S. Pat. No. 4,642,161. That is, it is believed that since a fine irregular surface of the copper oxide layer is obtained by the chemical oxidation, the adhesion strength is increased. However, there is a problem that the copper oxide (CuO) layer formed by the chemical oxidation is readily etched when a multilayer circuit board composed of thus treated internal and external circuit layers is dipped into a chemical plating solution or an electroplating solution including an acid to proceed a through-hole plating. That is, there is a possibility of causing a pink-ring around the through-hole plating. As higher integration of the copper conductors in the multilayer circuit board, a distance between the adjacent through-holes becomes smaller, so that the pink-ring is one of the most important problems to be improved for increasing a confidence of the multilayer circuit board. For overcoming the above problem, it is proposed that after the copper oxide (CuO) layer is formed on the copper conductors, the copper oxide (CuO) layer is reduced to metallic copper having acid insolubility, so that the pink-ring is effectively decreased. However, it is not possible to proceed a perfect reduction of the copper oxide (CuO) layer to metallic copper. Therefore, the reduction method is not always satisfied to perfectly prevent the pink-ring. By the way, in this specification, "oxidation" is defined as a formation of copper oxide in a surface of metallic copper by which a blackening of metallic copper is caused. Therefore, supply of hydroxy groups to the surface of the metallic copper is not included in "oxidation".